This stop can be described only by its purported purpose: to bind or "couple"
together the tones of other stops. To serve that purpose, a wide variety
of constructions and tones have been used.
The earliest examples, according to Grove and Williams, were open flue stops:
Principals,
Gemshorns or
Spillflöten.
In Habsburg Europe (16th, 17th & 18th centuries, esp. Austria and Germany)
it was a stopped flute, essentially a
Gedeckt, at 16', 8' and 4' pitches.
At various times and places this stop has also been a
mutation or a
Mixture of two or three ranks.

The function of the Coppel is often provided by stops of other names.
Irwin lists the following examples:

Variants

Examples

Osiris contains nineteen examples of Copula at 8' pitch and six at 4',
seven examples of Copel at 8' and one each at 4' and 16', seven examples
of Coppel at 8' and two at 4', and three examples of Copl at 8'.
The oldest ones are listed below.
No examples of Coupler, Coppeldoef, Coppelfluit, Koppel, Thunbass or
Coupling-Flute are known.